This invention relates to current-limiting devices and especially to current-limiting devices for high-current circuits.
Interest in the development of current-interrupting technology has increased recently due to new requirements in the power transmission industry and to realistic prospects of utilizing magnetic storage for production of high power pulses. The power transmission at increased voltages and use of d.c. power lines has created a need for opening switches with increased voltage and current capabilities and reduced opening times. The large energy storage systems with high power needed in the electron beam fusion experiments, for driving flash lamps in laser fusion experiments and in magnetically confined plasma experiments have stimulated the development of inductive storage systems. The key element in the inductive storage system, as in the case of the power transmission protection system, is the opening switch.
The development of the opening switch technology is aimed at interrupting currents reliably and reproducibly with precise triggering in addition to operating at higher current and voltage levels. In inductive storage systems, the high voltage stress on the current interrupting element and its high resistance are important because of their impact on the efficiency of the storage system. The development of these elements employs a wide variety of concepts and covers a broad set of operational parameters. Mechanical disruption of conductors using explosives to interrupt a current flow offers an attractive method for extending the technology of opening switches into the regime of modern power transmission systems and for use in large, efficient, pulsers based on the magnetic storage principles.
Mechanical disruption of current-carrying conductors using controlled separation of the electrodes, as in circuit breaking switch gear, is too slow for many applications. Therefore, more recently magnetic or explosive disruption of conductors has been employed to decrease the opening time from milliseconds to tens of microseconds. Magnetically driven pressure wave in oil has been used to interrupt 200 kA in 20 .mu.sec at 9 kV across a single gap. One disadvantage of magnetically operated interruptors is the need for a current source capable of operating at hundreds of kiloamperes. Use of explosives provides means of mechanically cutting a large series of gaps in tens of microseconds. The use of a series of gaps to achieve high switching voltage, as well as the capability for the precise triggering for initiating the opening action provides a high degree of versatility in switch design.
The basic concept underlying the mechanical interruption of current is the formation of an arc across the separating electrodes and subsequent cooling of the arc by a contact with cold gases, liquids, or solids in its immediate vicinity, or by temporary reduction in arc current by use of external circuits. Alternately, arc resistance can be used to limit currents to prescribed values. Cooling of the arc leads to increased arc resistance and its eventual extinction. In addition, the separating electrodes and structural elements of the switch, as well as gases and debris generated in the process of disruption must often withstand the inductive voltage generated across the switch. This is an important requirement which is not easy to meet since, in general, the peak voltage appears during the time when the electrodes and the pressure transfer (pusher) medium are still in motion. These are, therefore, two aspects of the opening switch design, mechanical and electrical, that must be taken into consideration together.